Currently paths are handled by `types.package`, whose semantics are a
bit of a mess. In particular, it converts path values to derivations
using `toDerivation`, which will lead to problems when flake `outPath`s
become paths in https://github.com/NixOS/nix/pull/6530.
This change makes the "incompatible changes" section in the above PR
obsolete: `nix.registry.nixpkgs.flake = nixpkgs;` works as expected (the
flake is copied to the store).
bcb5f0decc